if(conditional) {

Ok, sorry... you want to set it back immediately to 30, or to decrement it to 30 ?

No, maybe you did... 

If I got it right, opacity means that alpha argument for color and, by hit 100 opacity and back to 30, you meant bouncing between that range, right?
Well, I was doing an ellipse bounce animation sample for another thread. So, why not include an opacity bouncing as well?
Hope it can help you somehow:

1. //
2. // Ellipse Bouncing Animation Sample (v1.5)
3. // by GoToLoop (2012/Oct)
4. //
5. // https://forum.processing.org/topic/using-arrays-to-create-and-change-circles
6. // https://forum.processing.org/topic/opacity-loop
7. //
8. 
   
9. private static final byte ellipseTotal = 3;
10. private static final MyEllipse[] my = new MyEllipse[ellipseTotal];
11. 
    
12. final void setup() {
13.     size (500, 500);
14.     frameRate(60);
15.     smooth();
16.     noStroke();
17. 
    
18.     my[0] = new MyEllipse(
19.     120, 70, // initX, initY
20.     50, 40, // sizeX, sizeY
21.     7, 3, // movX, movY
22.     #0000FF, 30, 4); // colour, opacity, opacIncrease
23. 
    
24.     my[1] = new MyEllipse(
25.     320, 400, // initX, initY
26.     40, 50, // sizeX, sizeY
27.     3, 7, // movX, movY
28.     #00FF00, 100, -5); // colour, opacity, opacIncrease
29. 
    
30.     my[2] = new MyEllipse(
31.     250, 250, // initX, initY
32.     30, 30, // sizeX, sizeY
33.     -7, 4, // movX, movY
34.     #FF0000, 50, 1); // colour, opacity, opacIncrease
35. }
36. 
    
37. final void draw() {
38.     background(0);
39. //    for ( byte index=0; index<ellipseTotal ; my[index++].everything() );
40.     for ( MyEllipse ellip : my )    ellip.everything();
    
41. }
42. 
    
43. private final class MyEllipse {
44.     short x, y;
45.     byte  szX, szY;
46.     byte  mvX, mvY;
47. 
    
48.     color cor;
49.     short opac;
50.     byte  oInc;
51.     private static final short oMin = 30, oMax = 100;
52. 
    
53.     MyEllipse (
54.     int initX, int initY,
55.     int sizeX, int sizeY,
56.     int movX, int movY,
57.     color colour, int opacity, int opacIncrease) {
58. 
    
59.         x    = (short) initX;
60.         y    = (short) initY;
61.         szX  = (byte)  sizeX;
62.         szY  = (byte)  sizeY;
63.         mvX  = (byte)  movX;
64.         mvY  = (byte)  movY;
65.         cor  = colour;
66.         opac = (short) opacity;
67.         oInc = (byte)  opacIncrease;
68.     }
69. 
    
70.     private final void everything() {
71.         update();
72.         opacity();
73.         display();
74.     }
75. 
    
76.     private final void update() {
77.         x += mvX;
78.         y += mvY;
79.         if (x < 0 || x > width)    mvX *= -1;
80.         if (y < 0 || y > height)   mvY *= -1;
81.     }
82. 
    
83.     private final void opacity() {
84.         if (opac < oMin || opac > oMax)    oInc *= -1;
85.         opac += oInc;
86.     }
87. 
    
88.     private final void display() {
89.         fill(cor, opac);
90.         ellipse(x, y, szX, szY);
91.     }
92. }